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Basis for PSI variation depending on tire size?(5 posts)

Basis for PSI variation depending on tire size?PT
Aug 4, 2003 9:18 AM
Why are my 23c tires listed for 120 psi inflation and my 32c touring tires listed at 90 psi?

Assuming the 32c can handle 110 or 120 psi, would I get lower rolling resistance with the greater pressure?

I just want to understand the engineering/physical aspects. I won't buy into any comfort arguments -- I've spent too much time on mountain bikes not to intuitively understand the comfort and control issues of tire pressure, but I'd like to understand the efficiency issues as they're related to riding on pavement.
Pressure decreases as volume increases...biknben
Aug 4, 2003 9:42 AM
I'm sure someone can comment with the physics behind it but I'll give it to you from a dumbies perspective.

The measurement is in pounds per square inch. As tire size is reduced, you reduce the square inches of volume so the pressure increases as a result.
Non "dumbie" repsonse...MrDan
Aug 4, 2003 10:30 AM
Pressure x Area = Force.

Since the area of the tire casing increases with (surprise...) tire size, (I would "guess" that the area increases at some small exponential with respect to tire width.) the compressed air/gas generates more force trying to rip the tire out of the rim, and also stresses the casing itself. I'm sure you've been witness to what happens when you over-inflate... the tire itself doesn't usually get damaged, but the tube is sure shot!

Also with narrow versus wider tires... the amount of impact energy that can be absorbed (bump/pothole) without the tire pinching against the rim and flatting or damaging the rim decreases as the tire width decreases.

-D
force is actually linear, function of circumferencecyclopathic
Aug 4, 2003 2:38 PM
Correct statement otherwise you don't want tire to roll off the rim. 28mm at 100psi generates as much force as 23mm at 120psi.
MrDan's right . . .micha
Aug 4, 2003 10:44 AM
The larger the tire, the more force is generated at a given PSI. That's why I'd much rather stand next to a 120 PSI bicycle tire blowout than next to a 30 PSI automobile tire blowout.